An electrolytic ozone generator may produce ozonated water used, for example, for disinfecting purposes. More particularly, ozone is a strong oxidant that is used for water treatment and disinfection. In many applications, ozone replaces chlorine because of unwanted by-product formation connected with the latter. Ozone dissolved in water is used for disinfection of microbes and organic pollutants, wastewater treatment, and the like. The electrochemical production of ozone has advantages over the conventional technologies such as corona discharge. Ozone from electrochemical production is directly dissolved in water; thereby minimizing technical problems associated with handling ozone gas which is toxic at high concentrations.
Electrodes are a primary element used in the operation of the ozone generator. Electrodes used in prior art ozone generators often suffer from inefficiencies or high manufacturing costs due to size, materials of fabrication, and design constraints, such as geometry. Therefore, it would be beneficial to design electrodes to utilize the maximum electrode surface area and produce as much ozone as possible.
During water electrolysis, oxygen evolution is the main rival reaction to ozone production. Thermodynamically, oxygen evolution is strongly favored versus ozone production. Therefore, high current efficiencies for electrochemical ozone production are only possible for anode materials with a high overpotential for oxygen evolution. In the recent years, doped diamond electrodes have been developed and investigated for generation of dissolved ozone. Besides other interesting properties, doped diamond is distinguished by an exceptionally high overvoltage for oxygen evolution in aqueous electrolytes which makes even highly efficient OH radical production possible. In addition, diamond and related materials are stable in aqueous electrolytic processes.
According to an illustrative embodiment of the present disclosure, electrodes for use within an ozone generator include a plurality of plates made from electrically conductive material such as boron doped silicon, glassy carbon or oxidation resistant metals, such as titanium or niobium. In the illustrative embodiment, electrically conductive boron doped silicon is preferred due to its superior chemical resistance under anodic or cathodic operation, and its relatively low cost. Each electrode includes a front surface and a rear surface, the front surface coated with an electro-catalyst such as boron doped diamond, platinum, ruthenium oxide, or indium oxide. In the illustrative embodiment, boron doped diamond is preferred due to its high electro-catalytic activity and oxidation resistance. The electro-catalyst coating is supported by the front surface of the conductive plate, with a plurality of apertures formed within the conductive plate.
A hydrophilic electrolytic separator or membrane is illustratively disposed intermediate the front surfaces of the electrodes, forming the anode and cathode of the electrolytic cell. The separator is solid (i.e., without mating apertures) thus providing a liquid and gas barrier between the anode and cathode electrode apertures and preventing recombination of reactive oxygen and hydrogen species.
Oxygen and hydrogen are created at the anode and cathode respectively due to the electrolysis of water within and near the hydrophilic electrolytic separator. Electrolysis occurs at facing anode and cathode surfaces. Therefore, the electrode outer edges and the plurality of apertures must adjacently align to the mating electrode having substantially identical geometry.
The plurality of apertures in the electrode allows electrolysis products created at the mated surfaces of the electrode and separator to escape and allow the transport of replacement liquid water into the electrolytic separator. Maintaining hydration of the electrolytic separator is essential for ionic conductivity and the production of ozone. As such, careful sizing and placement of the apertures is necessary to maximize the surface area and allow the separator to rehydrate water during water electrolysis.
During electrolysis, rehydration of the separator must occur at or very near the mating exterior edges or aperture edges of the electrode. The summation of these edges of the electrode is collectively called the electrode Total Edge Perimeter. Due to the separator's structure some of the water necessary to rehydrate must enter into the separator near the electrode perimeter edges and travel along the interface between the separator and the electrode. This water can only travel a short distance along the mated surface interface, about 0.3 mm-0.4 mm, which is known as the Critical Offset Band. Therefore, to utilize the maximum surface area of the electrode for electrolysis, the perimeter edges of an electrode should lay within 0.6 mm-0.8 mm of other adjacent perimeter edges on the same electrode face, i.e. spaced 0.6 mm-0.8 mm apart (i.e., two times the 0.3 mm-0.4 mm Critical Offset Band). Distances greater than said Critical Offset Band do not allow sufficient water flow to rehydrate more distant areas of the separator, and therefore have very little electrolytic activity and ozone production.
The electrode surface area that contributes the majority of electrolytic reactions is collectively called the Active Surface Area. A majority of the electrolytic reactions occur at the interface of the separator and electrode as opposed to the Aperture Opening Surface Area or beyond the Exterior Edge Perimeter of the electrode. In order to utilize the electrode's maximum Active Surface Area from the available Tangent Surface Area, the size, shape and spacing of the apertures must be considered.
Gaseous products created during electrolysis are expelled through the apertures. Water necessary to rehydrate the separator is drawn in through the apertures. Apertures subtract from the available Active Surface Area used for electrolytic reactions.
To manufacture an ozone generator at the lowest cost it is advisable to optimize the Active Surface Area Current Density of the electrodes. Minimum electrode size is determined by the ozone generator operating life at maximum Active Surface Area Current Density of the electrolyte and electrode. Commercially existing solid polymer electrolytes generally limit operating current densities to less than about 1.5 amps/cm2 for 200 hrs of operation.
It is therefore desired to design electrodes that operate at the maximum Active Surface Area Current Density using the maximum Active Surface Area available from the Tangent Surface Area. The Tangent Surface Area is defined by the surface contact area between the front of the conductive plate and the separator, and represents the maximum possible surface area of the electrode for producing electrochemical reactions. A measure of how well the electrode utilizes the Active Surface Area compared to the Tangent Surface Area is the Active/Tangent Surface Area coefficient defined by the Active Surface Area divided by the Tangent Surface Area. Active/Tangent Surface Area coefficients nearing 100% are ideal.
The Critical Offset Band is defined where maximum electrochemical reactions occur from the Total Edge Perimeter. The Active Surface Area is determined by the numerical integration of non-overlapping Critical Offset Bands along the boundary edges of the front conductive plate. The Total Edge Perimeter is defined by the Outside Edge Perimeter of the front conductive plate summed with the Aperture Edge Perimeter. The Aperture Edge Perimeter is defined by the summation of perimeter edges of the plurality of apertures within the conductive plate. The Outside Edge Perimeter is defined by the total exterior perimeter of the front conductive plate.
An Active/Tangent Surface Utilization coefficient is defined by the Active Surface Area divided by the Tangent Surface Area, wherein said Ideal Offset Band is between 0.3 mm and 0.4 mm. Illustratively, the Active/Tangent Surface Utilization coefficient is at least equal to 90%, and most preferably 100%.
An Active/Gross Surface Utilization coefficient is defined by the Active Surface Area divided by the Gross Surface Area, wherein the Ideal Offset Band is between 0.3 mm and 0.4 mm. Illustratively, the Active/Gross Surface Utilization coefficient is at least equal to 60%, preferably greater than 70%.
A Perimeter/Aperture Ratio is defined by the Total Edge Perimeter divided by the Aperture Opening Surface Area. Illustratively, the Perimeter/Aperture Ratio is greater than 15 20 mm/mm2, preferably greater than 20 mm/mm2.
An Average Offset Band is defined by the Active Surface Area divided by the Total Edge Perimeter. Illustratively, the Average Offset Band is greater than 0.2 mm and less than 0.4 mm, and most preferably around 0.3 mm.
According to an illustrative embodiment of the present disclosure, an electrode for use within an ozone generator includes a conductive plate having a front surface and a rear surface extending between an outside edge perimeter, and a gross surface area defined by the front surface of the conductive plate within the outside edge perimeter. An electro-catalyst coating is supported by the front surface of the conductive plate. A plurality of apertures are formed within the conductive plate and define an aperture edge perimeter and an aperture opening surface area. A tangent area of the conductive plate is defined by the gross surface area less the aperture opening surface area. A total edge perimeter is defined by the outside edge perimeter of the conductive plate and the aperture edge perimeter of the plurality of apertures. An operative offset band is defined a predetermined distance from the outside edge perimeter and the aperture edge perimeter. An active surface area is defined by the operative offset band within the tangent surface area. An active/tangent surface utilization is defined by the ratio of the active surface area over the tangent surface area, the active/tangent surface utilization being greater than 90%.
According to a further illustrative embodiment of the present disclosure, an electrolytic cell for use within an ozone generator includes a cathode, an anode, and a hydrophilic electrolytic separator positioned intermediate the cathode and the anode. Each of the cathode and the anode includes an electrode having a conductive plate, a plurality of apertures formed within the conductive plate, a tangent surface area, an active surface area, and an active/tangent surface utilization defined by the ratio of the active surface area over the tangent surface area. The active/tangent surface utilization is greater than 90%.
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrate exemplary embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.